1. Field of the Invention
The present invention relates to a foil capacitor of the type comprising a block made by the stacking of capacitor-forming bands, for example metallized plastic films, the bands being appropriately connected mechanically and electrically by two output plates or electrodes, each plate being formed by a succession of metal layers deposited on the opposite flanks of the block.
2. Description of the Prior Art
The manufacture of stacked capacitors out of metallized plastic films is known. Plastic films having a metallized zone and a non-metallized lateral margin are generally used. Two films are superimposed with their non-metallized lateral margins on opposite sides so as to obtain a pair of films constituted by an even-order film and an odd-order film. At least one pair of metallised films is wound in a determined number of turns on a large-diameter wheel.
Thus, a capacitive strip, having alternating even-order and odd-order layers, known as a parent capacitor, is obtained. Each of the flanks of the capacitive strip is then covered with a metal (or alloy) in order to make output plates. Each output plate enables the metallizations of the same order layers to be connected electrically and mechanically to one another. This operation, which is done by the spraying of molten metal, is known as the Schoop's metal-spraying process.
To improve the results of this operation, it is recommended that there be a displacement of the even-order films with respect to the odd-order films, enabling the particles of the Schoop's process to be housed in order to obtain optimum clinging of same-order films. This displacement may be called an offset and is controlled during the operation for winding the films on the large-diameter wheel. Cutting the capacitive strip into blocks makes it possible to obtain distinct capacitors that are generally parallelepiped shaped. They are foil capacitors, and are generally constituted by a stack of alternating odd-order and even-order foils, held between two output plates located on the opposite flanks of the block. Connection elements are then fixed to the output plates of each capacitor.
There is also a known way of making the output plates in the form of a double metal layer having an internal layer adjacent to the metallized films and an external layer, the layers being made of different metal compositions. For, it often proves to be necessary to deposit an external layer, with a metal composition chosen for its excellent soldering properties, on an internal layer with a metal composition chosen for its excellent properties of adhesion, physical and thermal strength and electrical conduction.
However, the difference in the metal compositions used for the two layers of the plates embrittles the interface link between these two layers, notably in the event of quick variations in temperature. For, the layers have different coefficients of linear expansion, whence the appearance of cracks leading to a degradation (by increase) of the effective series resistance (E.S.R.) of the capacitor.